﻿54 BRITISH FOSSIL CRUSTACEA. 



Genus 1. — Pterygotus, Agassiz. — Continued. 



Species 2.— PTERYGOTUS BILOBUS :— Salter. 1859. 



1. var. a, inornatus. 



2. var. (5, crassus. 



3. var. y, perornatus. 



4. var. S, acidens. 



The species about to be considered was first described by Mr. Salter, under the 

 generic name Himantopterus, in the 'Quarterly Journal of the Geological Society' for 1855, 

 vol. xii, pp. 27 and 28, where a figure is also given (reproduced in ' Siluria/ last edition, 

 1867, p. 162). 



Of the species of Himantopterus, described in the paper above alluded to, two only at 

 present call for consideration, namely, H. bilobus and H. perornatus. Mr. Salter subse- 

 quently referred these to Pterygotus (see 'Mem. Geol. Surv./ Monograph 1, 1859, p. 39). 



At the time of the publication of the above Monograph comparatively but few entire 

 remains of Pterygotus bilobus had been obtained, whilst Pt. perornatus was even less 

 perfectly known from parts of specimens. 



It is probable that Mr. Slimon has obtained not fewer than several hundred specimens, 

 more or less perfect, of Pt. bilobus alone, since the commencement of his labours in the 

 shales of Lesmahago, but Pt. perornatus continues an extremely rare form. 



If a comparison be instituted between the figures in the first plate of the Geo- 

 logical Survey Monograph on Pterygotus (published in 1859) and the six plates which 

 accompany the present part of this Monograph, some idea may be formed of the progress 

 which has been made in the discovery of new materials, during the past nine years, for the 

 more complete elucidation of this remarkable genus. 1 



1 This progress will be still further illustrated by the genera Slimonia, Eurypterus, and Stylonurus, 

 the descriptions of which will follow the present part. 



